The present invention relates in general to broadcasting radio frequency signals to an enclosed area, and more specifically, to providing radio broadcast signals having a concentrated intensity which are particularly useful in testing radio systems during their installation into vehicles on an assembly line.
Wireless broadcasting from radio towers, such as in standard AM and FM broadcasting, transmits radio frequency (RF) signals through the air to individual receivers. The RF signals have a limited ability to penetrate into tunnels, buildings, and other structures. Receivers in these locations may be unable to receive a usable signal. Therefore, rebroadcast systems are used which employ an external antenna on the outside of the building and transmission-line wiring (possibly including an amplifier) for bringing RF signals inside the building without attenuation and then rebroadcasting with an internal antenna to receivers located in the structure.
It may also be desirable to provide only an internal antenna for a system broadcasting dedicated signals within a structure. In other words, the source signal for such a broadcasting system need not be externally derived radio broadcast signals. Nevertheless, in any such a system, it is important to restrict broadcast of signals to be within the structure and minimize external radiation which could interfere with other broadcasts outside the structure.
One application of rebroadcast type systems is in the testing of radio receivers and audio systems in automobile manufacturing plants. During manufacture of an automobile, antenna connections and speaker connections to the audio system must be checked. In a typical process, after installation of the radio and all of its interconnections, the radio is powered up and an operator presses the seek button to perform a seek tuning operation which stops at a received broadcast station of sufficient strength. If the radio fails to stop at any frequency (even though a sufficiently strong broadcast signal is present), then the antenna connection needs to be checked. Once a station is received, the audio is played through the speakers so that each speaker may be listened to, thereby permitting its speaker connections and proper operation to be verified (this process is often referred to as a speaker "walkaround" test).
A test area for performing these checks is typically inside a large building having a large amount of metal structure which results in highly attenuated RF signals penetrating the building. Furthermore, during the vehicle manufacturing process, a full radio antenna is typically not installed. In order to avoid antenna breakage during shipping of vehicles to their point of sale, only the antenna stub or base is present during manufacture. The full whip antenna is installed after shipping of the vehicle (e.g., at the dealer). Since only a partial antenna is present, the radio is even less sensitive to RF signals.
Typical rebroadcast systems in a building use a long-wire antenna inside the building which spreads the broadcast RF signals over a large area and fails to provide uniform transmission fields. Therefore, typical broadcast systems have had difficulty providing sufficient field strength for testing of radio systems in vehicle assembly plants. Furthermore, the exact location of radio testing on a vehicle assembly line may change from time to time, which may lead to problems due to the lack of uniformity in the broadcast field.